The invention relates to a gas burner for a Claus furnace, composed of at least five concentric tubes (T1-T5), forming five concentric spaces for the introduction of gas, the first tube (T1) being the tube of smallest diameter and the fifth tube (T5) being that of the largest diameter, in which:                the ends on the side facing in the Claus furnace of the third, fourth and fifth tubes lie in the same plane;        the two central tubes (T1, T2) are fastened together and able to move along the central longitudinal axis of the burner with respect to the other tubes (T3-T5), their ends on the side facing the Claus furnace not being able to pass beyond the plane formed by the ends of the three other tubes; and        the space formed between the first and second tubes (T1, T2) terminates, on the side facing the Claus furnace, in an injector oriented towards the periphery of the burner in the direction of injection of the gases into the burner.        
Depending on the ammonia content of the ammonia-containing gas to be treated in the Claus furnace, the two central tubes (T1, T2) are moved so as to obtain complete removal of the ammonia.
The present invention relates to a burner and to a partial oxidation process, in a Claus furnace, for a stream of gas comprising hydrogen sulphide and ammonia by reaction with a stream of oxygen-rich gas.
Gas streams rich in hydrogen sulphide are waste gases produced by many industries, especially the oil refining industry and the production of natural gas. Especially for environmental reasons, these gases rich in hydrogen sulphide cannot be released as such into the atmosphere. It is therefore necessary to treat them for the purpose of substantially reducing their hydrogen sulphide content. A process well known for treating these gases rich in hydrogen sulphide is the modified Claus process, commonly called the Claus process.
This process comprises a thermal part and a catalytic part. In the thermal part, two main reactions are carried out. The first reaction consists in reacting a portion of the hydrogen sulphide with oxygen in order to produce water and sulphur dioxide in the following manner:H2S+3/2O2H2O+SO2  (i)
By this first reaction, approximately ⅓ of the hydrogen sulphide to be treated is oxidized. The remaining ⅔ are reacted with the sulphur dioxide formed during the above first step, according to the following reaction, called the Claus reaction:2H2S+SO23/2S2+2H2O  (ii)
The combustion products are then cooled in a heat recovery boiler and then in a first condenser in which the elemental sulphur is recovered in liquid form. The gases are then reheated to a temperature allowing them to be treated on one or more catalytic beds (each of these beds being followed by a condenser). The Claus reaction continues on the catalytic beds until a hydrogen sulphide degree of conversion is obtained which is compatible with the standards governing the discharge of sulphur dioxide coming from the final step of the process, which is the incineration of the residual H2S. In the case in which two or three catalytic beds do not allow the sulphur dioxide discharge standards to be reached, a tail gas treatment unit may be added before the waste gases are sent to the final incinerator.
Refinery-treated gas streams rich in hydrogen sulphide may sometimes contain ammonia in addition to hydrogen sulphide. This is the case, for example, with the waste gases resulting from acid-water strippers in which the condensates of the processes (for example, the hydrocracking or catalytic cracking step in particular in the case of high hydrodesulphurization charges) are steam-stripped so as to recover the hydrogen sulphide and the ammonia. Typically, these gases are composed of one third hydrogen sulphide, one third ammonia and one third water vapour.
During the treatment of these gas streams by the Claus process, the destruction of the ammonia therein must be as complete as possible in order to avoid severe operational problems in the Claus unit. This is because, downstream of the heat recovery boiler, deposits of ammonium salts in the cold lines or on the output side of the condensers may cause blockages, degradation in the performance of the unit and eventually an increase in sulphur dioxide emissions. When the Claus process is being implemented, the ammonia may be destroyed by various chemical reactions (oxidation, thermal dissociation) which take place at the same time as the first reaction (i) of the Claus process.
It is recognized that destruction of the ammonia present in the gases containing hydrogen sulphide is favoured by a high temperature. This destruction may be implemented with Claus oxidation processes involving only air or processes involving both air and oxygen.
In oxidation processes involving only air, the treatment of the gases containing hydrogen sulphide and ammonia may be carried out:                either by using a two-zone furnace with by-pass of a portion or of all of the gases not containing ammonia. This solution makes it possible to increase the temperature of the first zone in which all of the gas containing ammonia is oxidized. Its drawback is that it can cause poor destruction of the hydrocarbon- or amine-type contaminants present in the gas not containing ammonia and create problems other than the deposition of ammonium salts (for example, coking of the downstream catalysts);        or by using refinery fuel gas to increase the temperature in the reaction furnace. The major drawback with this operation is the increase in the amount of gas passing through the unit, something which may result in a bottleneck. In addition, introducing refinery fuel gas into the Claus furnace has a tendency to increase the CS2 and COS contents in the case in which the fuel gas contains a great deal of CO2 in the gases coming from the Claus furnace, and this results in a lowering in the performance of the catalytic beds downstream of the furnace.        
In oxidation processes involving both air and oxygen, that is to say in which the combustion air is replaced with an air/oxygen mixture, it is possible to obtain better treatment of the gases containing hydrogen sulphide and ammonia, since the enrichment of the combustion air with oxygen increases the temperature in the reaction furnace and thus improves the destruction of the ammonia. However, in this case, not only is the temperature of the gases containing ammonia increased, but also that of the gases not containing ammonia; the amount of oxygen used is therefore not optimized. Furthermore, the temperature obtained is not always compatible with the metallurgical characteristics of the burner used.
To solve the problem of the use of oxygen specifically to increase the temperature of the gas containing ammonia, specific burners have been proposed which allow a separate feed:                for the air;        for the pure oxygen or for the oxygen-enriched air;        for the gases containing ammonia; and        for the gases not containing ammonia.        
Using these specific Claus burners, allowing separate confinement of the streams of the various gases, it has been possible to obtain hotter or cooler zones inside the flame. This allows localized temperature increases dedicated to the destruction of ammonia and makes it possible at the same time to maintain a “cooler” temperature for the oxidation of the other gases and in contact with the refractories of the furnace. This type of burner is disclosed, for example, in the applications EP-A1-0 810 974 and EP-A1-0 974 552.
The dimensioning of these Claus burners is carried out, among other characteristics, on the basis of a reference ammonia-containing gas having an average ammonia content and an average flow rate. The term “dimensioning” is understood essentially to mean the diameters of the tubes feeding the burner with the various gases. However, depending on the operating conditions of the refining units located upstream of the Claus unit (for example a severity of the hydrocracker or the hydrodesulphurization unit), the ammonium content of the gas to be treated may vary greatly, temporarily or otherwise, in relation to the content of the reference ammonia-containing gas. For example, for a refining site comprising a hydrocracker, depending on whether this hydrocracker is operating or not, the ammonia content of the gas to be treated may vary between 15 and 35%. This variation may also be due to poor operation of the acid-water stripping unit (for example malfunction of the condenser). The amount of oxygen to be injected in order to completely destroy the ammonia is then different and therefore the dimensioning of the Claus burner must be different if it is desired to obtain optimum removal of the ammonia in the Claus unit. However, to change the dimensions of the burner means changing the burner itself, something which cannot be envisaged for each appreciable change in the gas to be treated.